In wireless telecommunication networks, service providers are intensely interested in providing high quality, reliable services for their customers in today's high competitive marketplace. A significant aspect affecting the service quality is the consistency of radio coverage within cell coverage areas of the network. Moreover, an additional aim from the provider's perspective is to be able to increase capacity while maintaining quality and reliability. As known by those skilled in the art, telecommunication networks operating in accordance with code division multiple access (CDMA), which are also referred to as spread spectrum systems, the cell coverage is particularly affected by the traffic load in the cell. For example, as more traffic is carried by the cell, its coverage area tends to contract, on the other hand as less traffic is present, the coverage area of the cell tends to expand.
The tendency for cells to shrink and expand in relation to number of users in the cell is known in the art as "cell breathing" and occurs, for example, since each user in a CDMA system cumulatively contributes to the interference in the cell since they simultaneously share a common frequency band. It should be noted that there are typically multiple frequency layers upon which the users may operate on. The multiple layers permit service providers to add capacity while conforming to predefined frequency bands specified by the operating standard. The inherent nature of spread spectrum systems permits all users to transmit and receive on the same frequency thus each of the transmissions necessarily "interfere" with each other. As more traffic appears in the cell, the more interference is introduced thereby increasing the power threshold that a mobile must transmit to overcome the interference in order to sufficiently communicate with the base station. This effect tends to be more prominent on uplink transmissions from mobiles since their power levels tend to be more limited in comparison to that of the base station.
An undesirable consequence of cell breathing is the development of coverage holes (or gaps) that may occur between cells during periods of high traffic load. Consequently, a mobile straying into a coverage hole may not have a sufficient connection to continue the call, thus the call may be dropped. In the context of the present invention, the term "call" is used interchangeably to include either voice or data traffic. The extent to which coverage holes develop in generally related to the cell planning performed by the service provider. By way of example, the provider typically uses cell planning tools, network measurements, field and drive tests among other things in order to determine a suitable base station deployment for sufficient network coverage. Thus, one known way of minimizing the undesirable occurrence of holes is to simply increase the number of base stations for a given network coverage area. Increasing the density of base stations permits sufficient coverage overlap in order to compensate for cell contraction due to cell breathing. However, a major disadvantage of adding more base stations is that it is an extremely expensive solution to implement in order to counter the effects of cell breathing.
Another technique that has been used in the prior art for reducing the likelihood of coverage holes is to carefully limit the amount of interference in the cell before it reaches precarious levels. Typically this is performed by admission control whereby a strict limit for traffic capacity is imposed within the cell. A theoretical load limit for ideal conditions may be calculated which thereby represents the capacity on the cell at 100% load. However, the practical load limit, which is the capacity level at which the development of coverage problems become unacceptable, is determined by using various theoretical and experimental methods including cell planning tools, network measurements, and field and drive tests. For example, a practical load limit of 65% on the cell may be found to be the point at which coverage holes start to become unacceptable. This may be reflected in a dropped call rate that is approaches unacceptable levels. Therefore, in most cases admission control algorithms are programmed to maintain the capacity levels of cells to stay within the practical load limit. It should be noted that the admission control level can be represented in other ways such as a specific number of users in a cell, for example.
The specific load limit levels may vary from network to network depending on the particular network configuration, for example, number of base stations, traffic volume, type of traffic i.e. voice or data etc. Hence a network operating with a strict form of admissions control may at times, and perhaps unnecessarily, limit the capacity in cells below a higher level that it may otherwise be able to handle, thereby depriving service providers of additional revenue. In view of the foregoing, it would be desirable to implement an improved traffic management technique that minimizes the development of coverage problems without unnecessarily limiting the overall network capacity.